1. Field of the Invention
The present invention relates generally to the field of fluidized bed combustor and reactor wall construction and in particular to a new and improved tube wall configuration for protecting and reducing erosion of tube walls in the lower regions of a fluidized bed combustor or reactor.
2. Description of the Related Art
In a circulating fluidized bed reactor, reacting and non-reacting solids are entrained within the enclosure by the upward flow of gases which carry some solids to the reactor exit at the upper end of the reactor. Other, larger quantities of solids are recycled within the reactor enclosure as heavier solids initially carried upwards fall back against the flow of gases. Since the velocity of the upward flow of gases is often much lower adjacent the circulating fluidized bed enclosure walls and heat transfer surfaces within the circulating fluidized bed, most of the solids fall near the walls or heat transfer surfaces.
The amount of solids falling adjacent to the walls and surfaces increases progressively toward the bottom of the circulating fluidized bed. The density of the bed is higher in the lower regions of the furnace, and as a result, the walls and surfaces in the lower regions are subject to increased erosion from contact with the solids.
Further, the reactions occurring in the circulating fluidized bed create chemical reduction conditions against which the walls and heat transfer surfaces must be protected. A protective material (further called refractory) is often used to coat the walls and exposed surfaces in the lower regions of the circulating fluidized bed. The refractory is expensive, since it must withstand high temperatures (between 1400.degree. and 1800.degree. F.), contact erosion from solids, and chemical reduction and by-products from the reaction. The refractory also reduces the efficiency of heat transfer. For this reason refractory is only applied to the walls and exposed surfaces to as low an elevation in the reactor region as possible considering corrosion and erosion conditions. At the point on the walls and surfaces where the coating terminates, a discontinuity is formed where erosion occurs. The erosion is typically in a band about 1/4" wide adjacent the top edge of the protective material.
One method of reducing the effect of this erosion is to place a weld overlay from adjacent the discontinuity to several inches above the protective material termination. The weld overlay thus protects the tube wall or heat transfer surface from erosion at the point of discontinuity. Unfortunately, the weld overlay also suffers from erosion and must eventually be replaced, usually at great expense.
Another method of reducing the erosion is shown in U.S. Pat. No. 5,239,945, which discloses a fluidized bed boiler or reactor having water walls which are sloped outwardly from the top, forming a trapezoidal enclosure. Since the furnace walls slope continuously outward, particles falling from the upper regions of the enclosure cannot directly impinge the walls at the line of the tube-refractory interface, and instead fall primarily downward into the bed.
A second wall configuration is disclosed by U.S. Pat. No. 5,091,156, which has a configuration of water walls in a fluidized bed for protecting the refractory lined portion of the walls from erosion by contact with particles in the bed. The initially straight enclosure walls in the upper region are first bent outward adjacent the refractory portion, and then bent a second time inward toward the fluidized bed and opposing wall. The protective refractory coating lines the water wall beginning at the first outward bend such that a smooth transition between the refractory and upper water wall is created.
This wall configuration does not entirely protect the refractory lining from erosion by contact, since the discontinuity is still exposed to downwardly falling particles which may impact the transition at a slight angle. And, neither of the last two wall configurations can be applied to walls or heat transfer surfaces located entirely within the circulating fluidized bed enclosure, such as internal walls.